This invention relates to a portable test instrument capable of rapid evaluation of impairments that affect data transmission on the voiceband circuits.
The subject of impairments to nonvoice signals on common carrier telephone channels, both switched and dedicated, has been thoroughly described in the following Bell System Technical references:
PUB 41009 entitled "Analog Parameters Affecting Voiceband Data Transmission-Description of Parameters" October, 1971; PA1 PUB 41008 entitled "Transmission Parameters Affecting Voice Band Data Transmission-Description of Parameters," July, 1974; and PA1 PUB 41009 entitled "Transmission Parameters Affecting Voice Band Data Transmission-Majoring Techniques" May, 1975. PA1 G(f.sub.o) and .phi. (f.sub.o) are the channel amplitude and phase characteristics respectively at frequency f.sub.O, PA1 n(t) is the total uncorrelated interference, PA1 m(t) is incidental AM, and PA1 .theta.(t) is incidental PM [d.theta.(t)/dt is incidental FM] PA1 1. Band limit around a carrier frequency (nominally 1010, .+-.300 Hz). PA1 2. Amplify and amplitude limit the carrier to strip off the A.M. PA1 3. Detect zero crossings jitter from the error voltage of a phase lock loop. PA1 4. Display filtered jitter (up to about 300 Hz) on a peak-to-peak indicating meter. PA1 (1) Phase jitter and amplitude jitter (both standard plus low frequency); PA1 (2) Phase jitter and comparison of standard and standard plus low frequency; and PA1 (3) Amplitude jitter in comparison of standard and standard plus low frequency.
Each signal format for nonvoice signals such as one or more tones, amplitude and/or phase modulated by either analog or digital information, is impaired to varying degrees by physical limitations, inevitable interferences, and design limitations. The subject invention will provide for the quick and efficient evaluation of impairments such as phase and amplitude jitter, both with and without low-frequency contribution and will simultaneously count phase hits, gain hits, drop outs and three levels of impulse noise.
For background information, incidental modulation has been defined as any unwanted AM, PM or FM imposed on the information carrying voiceband signal by a disturbing source other than itself.
Incidental modulation has been expressed in the above mentioned 1974 Bell System Technical references as the following summation: EQU V.sub.r =A.sub.O G(f.sub.o)[1+m(t)] cos [2.pi.f.sub.o t+.phi.(f.sub.o)+.theta.(t)]+n(t)
where
As suggested above, this invention relates to a test instrument which measures phase and amplitude jitter and simultaneously counts phase hits, gain hits, drop outs and three levels of impulse noise. The phase jitter may therefore be defined as undesired phase modulation on a received signal and typically results from unwanted phase modulation on carriers supplied in carrier terminals. The modulating frequencies are often harmonics of 60 Hz or other very low frequencies that are not easily filtered in power supply circuits. In making phase jitter measurements, the normal procedure is for a holding tone to be transmitted over the facility under test. The phase jitter measured at the receiver is the summation of any incidental phase modulation and random or quantized noise encountered on the facility. Actually all AC components of incidental phase modulation, which cause the zero crossings of a voice-band signal to "jitter" are measurements of interest.
Incidental amplitude modulation or amplitude jitter on a telephone channel takes the form of low index double side band modulation of voice band signals.
Finally, phase hits, gain hits and drop outs are phenomena which can be classified as abrupt changes in the phase or amplitude of a received sinusodial wave and which occur so infrequently, or exist for such short periods of time, that they cannot be properly assessed by any other known techniques. Known prior art instruments which have been used to measure gain and phase hits, as the rapid gain and phase changes are usually called, do so by monitoring the magnitude and phase of a sinusodial tone. Hits are usually recorded and accumulated on electromechanical counters with adjustable threshold levels.
Drop outs are short interruptions in service where the transmitted signal experiences the sudden large drop in power wherein the signal often becomes undetectable.
As has been described in the Bell System Technical Reference entitled "Analog Parameters Affecting Voiceband Data Transmission-Description of Parameters," October, 1971, there are suggested designs for instruments capable of measuring both phase jitter and incidental amplitude modulation. For example, the recommended design of phase jitter instruments includes the processing of a received voice frequency tone as follows:
Since incidental AM or amplitude jitter is low index and only small peak-to-peak excursions of the carrier are evident, it may become different to observe by an oscilloscope or envelope detection and to further distinguish means from additive uncorrelated interference. Accordingly, the suggested instrumentation of amplitude jitter detection and metering is similar to that for incidental phase modulation or phase jitter, and includes the process of band limiting and removal of other interfering modulation. A true peak detector, which when used in conjunction with an accurately controlled oscillator in a feed back loop, would then be capable of detecting only amplitude modulation on the carrier tone.
Known prior art instruments are generally designed along the line outlined above and in the above Bell System Technical References. Such instruments have further been disclosed in the two patents to Frank R. Bradley, U.S. Pat. Nos. 3,814,868 and 3,906,173, entitled "Telephone Line Characteristic Measuring Instrument" and "Telephone Line Characteristic Measurement Instrument and Display," respectively. The known patented devices generally utilize the Bell System approach, or parts thereof, and incorporate an oscilloscope for identifying the source of disturbance, e.g. either amplitude modulation or phase modulation, phase hits or white noise, etc. but require a relatively sophisticated ability to interpret the oscilloscope trace both in terms of type and quantity measurement.
The subject invention is designed to provide for the rapid elevation of impairments in that it will measure phase and amplitude jitter, both with and without low frequency contributions and simultaneously count phase hits, gain hits, drop outs and three levels of impulse noise. Phase and amplitude jitter are displayed on two linear bar-chart type readouts that are calibrated in degrees and modulation percent. Phase hits, gain hits, drop outs and impulse noise are counted simultaneously in parallel counting registers and the cumulative real time count total of each register is displayed on individual three digit numerical displays. The linear bar-chart displays are designed to indicate jitter in three combinational operating modes. They are:
To amplify briefly on the above, and considering mode number 1, the basis of both display readings is "standard plus low frequency." Thus, all jitter components in the 4 to 300 Hz range are measured. The upper display on the front panel of the instrument indicates phase jitter in degrees, peak-to-peak and the lower display indicates amplitude jitter in "equivalent degrees," peak-to-peak. By definition phase jitter measurements indicate the cumulative effect of incidental (true) phase modulation and additive tones or noise on zero crossings of a holding tone. Similarly, amplitude jitter includes both incidental amplitude modulation, plus the effect of additive tones or noise on the peak-to-peak value of the holding tone. It has been seen that there is little evidence of incidental amplitude modulation in a telecommunication facility, therefore a reasonable assumption can be made that the amplitude jitter display normally will show only the effect of additive tones or noise present on the circuit. If the two readings (upper and lower displays) are approximately equal, the problem is therefore likely to be single-frequency interference or noise since both equally affect phase and amplitude. On the other hand, a large phase-jitter reading combined with a small amplitude-jitter reading typically indicates the presence of both noise and incidental phase modulation. The noise component is responsible for the amplitude jitter reading and an equivalent portion of the phase jitter reading. In this case, the difference between the two readings is caused by pure phase modulation.
In mode 2, mentioned above, the contributions of low frequency phase jitter is identified. The upper display shows jitter components in the "standard" (20-300 Hz) range. The lower display shows jitter components in "standard plus low frequency" (4-300 Hz) range. Differences in the two readings are therefore directly attributable to low frequency phase jitter.
Mode 3 or amplitude jitter provides the same capability for isolating low-frequency amplitude jitter components. In this case both displays are calibrated in percent modulation. The upper display measures "standard" and the lower display measures "standard plus low frequency" components. As indicated above, the low incidence of true amplitude jitter will typically result in little variation between the two displays.
In addition to the above, the unit includes a unique audio monitoring scheme and an auxiliary output which permits the instrument to be interconnected with the X and Y deflections of an ordinary oscilloscope.
An object of the subject invention is to provide a new and improved method and telecommunication test instrument that is easy to read in that it is capable of displaying dynamic information yet at the same time retains certain advantages of digital computation.
Another object of this invention is to provide a method and unique telecommunications test set of the character described, a digital technique for accumulating phase and amplitude jitter; however, presenting such information in an analog type display.
A further object of the invention is to provide a method and unique telecommunication test set wherein certain related signals are visually compared so that the test instrument operator may see in "real time" the impairments of signal phenomena relative to each other. For example, by observing signal fluctuations simultaneously it is possible to determine whether the impairments are occurring in the low frequency (4 to 20 Hz) portion of the jitter spectrum or in the standard (20 to 300 Hz) frequency range. Accordingly the problem of visually detecting whether or not fluctuations and excursions are random in only one mode has been obviated and the simultaneous visual comparison of both modes provides meaningful information.
A still further object of the invention is to provide a unique telecommunication test set that provides for the simultaneous and separate display of phase hits, gain hits, drop outs and impulse noise. In this manner the instrument operator may visually observe excursions or perturbations on either of the linear bar-chart displays and immediately determine whether the perturbation was classified as a phase hit, a gain hit, a dropout or as impulse noise.
A further object of the invention is to provide a telecommunication test instrument of the character described immediately above which is coupled with an audio output capable of identifying some, or all, of the impairments (excursions, perturbations etc.) by the character of the sound emanating therefrom.
A further object of the invention is to provide a telecommunications test instrument having a unique display that is capable of facilitating the comparing of noise and making a determination of whether or not the impairment is noise or phase jitter. For example, if the linear bar-chart type display utilized herewith indicates equal magnitudes of both amplitude jitter and phase jitter, the instrument user can assume that the impairment is due to noise and not to incidental phase modulation.
A significant and important object of the invention is to provide a simplified and easy to read meter type instrument that requires a minimal amount of training for operation and diagnosis of impairments on voice bands of telephone circuits.
A further object is to provide a unique apparatus for converting digital information into an inertialess analog type display.
Other and further objects of the invention, together with the features and novelty appurtenant thereto, will appear in the course of the following description.